The invention relates to a cathode ray tube comprising an electron gun, a display screen and a deflection unit for deflecting the electron beam, which deflection unit includes a line deflection coil system and a frame deflection coil system.
The invention also relates to a deflection unit for use in a cathode ray tube.
Cathode ray tubes are employed, inter alia, in display devices such as television receivers, computer monitors and oscilloscopes.
In operation, the means for generating an electron beam generates one or more electron beams. The deflection unit generates electromagnetic fields for deflecting the electron beam (or electron beams) across a display screen in two mutually perpendicular directions. These directions are commonly referred to as the line direction (generally the horizontal direction) in which direction the display screen is scanned at a relatively high frequency, and the frame direction (generally the vertical direction) in which direction the display screen is scanned at a relatively low velocity. During deflection of the electron beam(s), a phenomenon occurs which will hereinafter be referred to as "ringing". A sudden change of the magnetic deflection field generated by the line deflection coil system causes an excitation of the line deflection coil system and/or the frame deflection coil system. This phenomenon occurs, in particular, during flyback of the line deflection and causes a deviation in the frame deflection direction on a line written in the line deflection direction and/or in the velocity at which a line is written in the line deflection direction. This deviation is visible, in particular, in an area at the edge of the display screen, that is the location where line scanning of the display screen starts.
A known measure for reducing this problem is a so-called overscan of the display screen. An overscan of the display screen means that line scanning starts some distance beyond the display screen. As a result thereof, ringing is not reduced but the consequences of this phenomenon are less visible or invisible on the display screen. This measure has the drawback that the velocity at which information is displayed on the display screen is increased and that the electron beam(s) must be deflected through a larger angle, so that more energy has to be supplied to the deflection system.